The present invention relates to electronic data displays, and more specifically concerns improvements in displays for presenting both alphanumeric (A/N) and graphics data simultaneously.
Alphanumeric text displays are burgeoning throughout the marketplace, and current dot-matrix technology can provide high-quality character images for such displays. Graphics data displays, long used in specialized areas, are rapidly broadening their applications base. Current bit-mapped graphics displays can provide increasingly better images for fields such as business presentations which previously had little or no entree to computer-generated graphics aids. The merger of applications which can use both text and graphics information creates a demand for a single display system which can present both A/N and graphics data on the same screen at the same time.
In the prior art, display units employed several approaches to the simultaneous presentation of A/N text and graphics on the same display.
Mosaic graphics uses entire character areas ("boxes") as single units. Each character is used as one or a small number of on/off picture elements ("pixels"). Typically, an eight-dot-wide by fourteen-dot high character box is divided into four pixels, each four dots wide by seven dots high; sixteen character codes are then allocated to all possible on/off combinations of these pixels. Character codes may also be used to define line characters, non-alphanumeric characters composed of single and doubled strokes for horizontal and vertical lines, intersections, corners, and so on, which can be combined with other such non-alphanumeric characters to form larger images. Such images are usually quite crude, since either the overall pixel size is large or the number of available line characters is small. This approach is commonly used in A/N displays to accommodate a limited graphics capability.
Bit-mapped characters treat text characters as merely another image in a dot-matrix graphics display. That is, an A/N character is drawn on the display along with the graphics shapes and is stored in the frame buffer as a shape having no particular significance as textual information. Several problems dog this approach. The major problem is that the ratio of optimum dot size for the A/N characters to optimum dot size for the graphics data is rarely 1:1. Nor is this ratio any other whole integer such as 2:1 or 3:1; non-integral ratios are much more desirable.
In a typical cathode-ray-tube (CRT) display, 720 A/N dots per horizontal raster scan are necessary to provide acceptable resolution for a line of eighty characters. Memory capacity is only a minor consideration for A/N dot size, because the size of the A/N buffer is independent of the actual number of dots in the character patterns, and the size of the ROM in the usual character generator is well within the capacity of a single inexpensive integrated-circuit chip. On the other hand, 480 dots per scan may be perfectly adequate for presenting graphics data on the same display. One of the majors factor limiting the number of graphics dots per scan is the capacity of the graphics buffer; going from 480 dots to 720 dots per scan--i.e., from the non-integral 3:2 ratio to an integral 1:1 ratio of A/N dots to graphics dots--would require a 1/3 larger graphics buffer capacity. Decreasing the number of A/N dots to 480 to achieve an integral 1:1 ratio would allow only a six-dot character width, an unacceptable loss of resolution.
Another factor limiting the number of graphics dots is the execution time for graphics programs; computational complexity and time is at least linearly proportional to the number of dots to be manipulated in a bit-mapped display. For this reason also the number of graphics dots in a scan line should be smaller than the number of A/N dots. But integral ratios such as two or three A/N dots per graphic dot--360 or 240 graphics dots per line in the preceding example--usually give unacceptably coarse graphics. In other words, the optimum dot-length ratio for most applications of combined graphics and A/N displays is between 2:1 and 1:1.